A number of chemical reactions, in particular exothermic reactions, e.g. direct synthesis of hydrogen peroxide H2O2 from the gaseous starting materials oxygen O2 and hydrogen H2, result in schematic problems which have to be taken into account when implementing said reactions or which place limitations thereon. In particular, undesirable side reactions are to be expected that are produced by unfavourable, in particular inhomogeneous, concentration ratios of the starting materials and reaction products in the reaction chamber. In the case of H2O2 direct synthesis, this inhomogeneity makes it easier for explosive gaseous mixtures to form which can lead to dangerous operating conditions in a system. A further problem relates to mass transport resistance in flows, in particular when gases are intended to be dispensed into liquids. Depending on the system, the effects increase as the associated reaction volume used for the reaction in each case also increases.
Proceeding from such limitations and risks, a reactor system is for example described in DE 10 2005 015 433 A1, in which mixing and a subsequent reaction are separated into a plurality of micromixers or microreactors connected in parallel. The processes in the reactor system are thus divided into smaller units that can be controlled more efficiently, meaning that not only does the temperature control during the reaction and the connection in parallel increase the reaction conversion that can be produced, but the risk of producing inhomogeneity and explosive gaseous mixtures as set out above can also be reduced.
Furthermore, a microstructure reactor is described in DE 100 44 526 A1, in which two starting materials are directed into groups of microchannels arranged in parallel with one another. In this process, one starting material is introduced into the flow of the other starting material at different locations and thus at different intervals via a plurality of overflow openings arranged behind one another, resulting in a gradual reaction in the flow in the microchannels to form a flow of reaction product. The concentration of one of the two reactants increases along the length of the reaction product flow path, while the concentration of the other reactant decreases. Adding one starting material to the other flow of starting material at different locations and thus at different intervals makes it possible to control the reaction between the two starting materials more efficiently.
Direct synthesis for producing hydrogen peroxide from the starting materials H2 and O2 represents a particular challenge in the experiment. By way of example, DE 196 42 770 A1 discloses direct synthesis of this type for producing hydrogen peroxide, in which the gaseous starting materials H2 and O2 are converted into liquid H2O2 using a catalyst. The reaction is always carried out in the presence of a solvent in which at least one noble metal, in particular palladium, acting as the catalyst is suspended. The risk of explosion is avoided by the H2 concentration in the gas phase being limited to 5% at most. The described reactor comprises a trickle bed in which the thin liquid film is saturated by means of the solid catalyst by said film being in open contact with the gas phase.
Membrane reactors have been proposed for direct synthesis of hydrogen peroxide which distribute an intake of the starting materials into the reaction and increase the safety and the selectivity of the method. Thus, Choudhary et al.: Angew. Chem. Int. Ed. 40 (2001), 1776-1779 describes using impenetrable membrane layers consisting of palladium alloys, by means of which the H2 required for the reaction was added to a liquid that was saturated with oxygen. In this process, the reaction took place on the liquid-side surface of the palladium membrane, to which surface various layers were applied for this purpose in order to increase the selectivity of the reaction.
Furthermore, by way of example, WO 2007/028375 A1 describes porous membranes into which palladium as a catalyst in the form of metal nanoparticles was introduced for direct synthesis of H2O2. In this concept, either H2 or O2 is added in a bubble-free manner to the reaction solution that is saturated with the other starting material via a porous membrane. The conversion takes place inside the cover layer of the porous membrane in which the catalyst is located.
The variants described in WO 2007/028375 A1, in which the hydrogen or the oxygen is fed along the reaction zone via tubular membranes, are subject to a comparatively high mass transport resistance (mass transfer of gas/liquid and diffusion in the liquid phase) caused by the relatively large inner diameter of technically standard membranes. Furthermore, in the proposed method, the conversion factor achieved when passing through the reactor is limited by the gas solubility under the conditions in the upstream saturator, and therefore, at tolerable system pressures, recirculation is required in order to produce concentrations of hydrogen peroxide (>3 wt. %) that are within the range of interest. This is relatively technologically complex. Additionally, only some of the fed oxygen is converted. The rest of the oxygen is lost when the pressure is relieved before the liquid phase is recirculated into the saturator, or said oxygen has to be retrieved from the gas flow, which is a considerably complex process.
U.S. Pat. No. 7,067,103 B2 and U.S. Pat. No. 7,105,143 also disclose catalytic reactors for directly synthesising hydrogen peroxide by gradually adding H2. The purpose of this gradual addition is to keep an O2:H2 concentration ratio along the reactor within an optimal range in a controlled manner, and at the same time to minimise the amount of unconverted O2 that has to be separated from the reaction once again and recirculated. Gradually adding H2 upstream of a reactor segment of a reactor cascade in each case is disadvantageous in that there is a possibility of explosive gaseous mixtures forming at the addition points and downstream thereof.
Bortolotto et al.: Sep. Purif Sci. 73 (2010), 51 and Dittmeyer et al.: Appl. Catal. A: General 391 (2011), 311 also describe reactor designs that use different membranes in order to add two different starting materials into microchannels in a distributed manner in order to optimize the concentration ratio thereof along the reactor axis for conversion on a catalytically active surface. Both starting materials are added in a distributed manner in the same reactor portion via permeable walls arranged opposite one another. However, in these designs, the gaseous starting materials are not separated. By using microstructures, the risk of explosion is indeed reduced; however, it is impossible to assume inherent safety, particularly when the system pressure is comparatively high for direct synthesis of hydrogen peroxide.